As used herein, the term “oil country tubular goods” refers to oil well steel pipes, for example, described in the definition column of No. 3514 of JIS G 0203 (2009). Specifically, the “oil country tubular goods (hereinafter abbreviated as OCTG)” means a general term for pipe and tube products such as casing, tubing, and drilling pipes which are used in drilling of oil wells or gas wells, and extraction of crude oil or natural gas.
As low-corrosive wells (oil wells and gas wells) have been exhausted, wells with high corrosiveness (hereafter, referred to as highly corrosive wells) has been developed. A highly corrosive well contains large amounts of corrosive substances. Examples of corrosive substance include corrosive gasses such as hydrogen sulfide and carbon dioxide gas, and the like. Hydrogen sulfide causes sulfide stress cracking (hereafter, referred to as “SSC”) in high strength and low alloy OCTG. On the other hand, carbon dioxide gas deteriorates carbon dioxide gas corrosion resistance of steel. Therefore, high SSC resistance and high carbon dioxide gas corrosion resistance are required for OCTG for use in highly corrosive wells.
It is known that chromium (Cr) is effective for improving the carbon dioxide gas corrosion resistance of steel. Therefore, in wells containing a large amount of carbon dioxide gas, martensitic stainless steels containing about 13% of Cr typified by API L80 13Cr steel (Conventional 13 Cr steel) or Super 13 Cr Steel, dupulex stainless steels, and the like are used depending on the partial pressure and temperature of carbon dioxide gas.
However, in a martensitic stainless steel and a duplex stainless steel, SSC attributable to hydrogen sulfide is caused at a lower partial pressure (for example, not more than 0.1 atmosphere) compared with in a low alloy steel. Therefore, these stainless steels are not suitable for use in environments containing large amounts of hydrogen sulfide (for example, environments where the partial pressure of hydrogen sulfide is not less than 1 atmosphere).
Japanese Patent Application Publication No. 2000-63994 (Patent Literature 1) and Japanese Patent Application Publication No. 07-76722 (Patent Literature 2) propose a steel which is excellent in carbon dioxide gas corrosion resistance and SSC resistance.
Patent Literature 1 describes the following matters regarding a Cr-containing steel pipe for oil wells. The Cr-containing steel pipe for oil-wells consists of, by mass %, C: not more than 0.30%, Si: not more than 0.60%, Mn: 0.30 to 1.50%. P: not more than 0.03%, S: not more than 0.005%, Cr: 3.0 to 9.0%, and Al: not more than 0.005%, with the balance being Fe and inevitable impurities. Further, the Cr-containing steel pipe for oil-wells has a yield stress of 80 ksi class (551 to 655 MPa).
Patent Literature 1 also describes that the above described Cr-containing steel pipe for oil-wells exhibited a corrosion rate of not more than 0.100 mm/yr in a carbon dioxide gas corrosion test at a carbon dioxide gas partial pressure of 1 MPa and a temperature of 100° C. Further Patent Literature 1 describes that in a constant load Lest conforming to NACE-TM0177-96 method A, the above described steel pipe showed no SSC under an applied stress of 551 MPa in a test Solution A (pH 2.7).
Patent Literature 2 describes the following matters regarding the production method of a martensitic stainless steel for OCTG. A steel mainly composed of martensite, and containing, by mass %, C: 0.1 to 0.3%, Si: <1.0%, Mn: 0.1 to 1.0%, Cr: 11 to 14%, and Ni: <0.5% is prepared. The steel is heated to a temperature between Ac3 point and Ac1 point, and is thereafter cooled to Ms point or lower. Thereafter, the steel is heated to a temperature not more than the Ac1 point, and thereafter is cooled to ambient temperature. This production method performs a duplex region heat treatment between quenching and tempering treatments. The steel produced by this production method has a yield strength of as low as not more than 50 kgf/mm2 (490 MPa, 71.1 ksi).
In general, in a carbon steel and a low alloy steel, the lower the strength, the more excellent the sulfide stress cracking resistance is, and it is considered that the same applies to the case of martensitic stainless steels. It is not possible to obtain a yield strength of steel of not more than 55 to 60 kgf/mm2 (539 to 588 MPa, 78.2 to 85.3 ksi) by a conventional heat treatment method of steel (method of performing normalizing and tempering). In contrast to this, the production method according to Patent Literature 2, which involves heat treatment in a duplex region, can obtain a low yield strength. Thus, Patent Literature 2 describes that the steel obtained by this production method is excellent in the SSC resistance and the carbon dioxide gas corrosion resistance.